I had a similar situation on my '51 AD where I had begun to get gear oil on one rear brake. After driving it and having one wheel drag, I pulled the drum and saw oil all over. Curious as to whether I had lost a lot of oil I opened the fill plug on the differential and to my surprise, got a puff of air (I had just returned from a drive and the diff was warm). Of course, that caused me to examine the axle vent, where I found it was completely sealed by crude and corrosion. I cleaned it up with a little gun solvent and then cleaned up my shoes and drums with brake solvent and have never had a problem since. I didn't have to replace seals or shoes.
The vent is just a 1/8 pipe thread on my AD. Check it out, maybe you can get lucky, too, and your seals might be OK.
Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix